(1) Field of the Invention
The present invention relates to the injection of pulverized materials into an enclosure wherein a high pressure, which is subject to variation, is maintained and particularly to the delivery of combustible solid material to the interior of a furnace without disturbing the desired furnace operating conditions. More specifically, this invention is directed to apparatus for supplying finely divided solid fuel to a shaft furnace through its tuyeres. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use in the delivery of pulverized coal to the interior of a pressurized shaft furnace of the type employed in the reduction of ore to produce cast iron. Such shaft furnaces are conventionally provided with a circular exterior conduit which is connected to the base of the furnace by a plurality of tuyere stocks and associated injection nozzles. The arrangement of circular conduit, tuyere stocks and injection nozzles accomplishes the injection of hot air blasts into the furnace. The heated air supplied to the circular conduit is produced in an installation comprising a booster (compressor), a set of cowpers (also known as hot-blast stoves) and a mixing station. A tuyere stock is depicted in U.S. Pat. No. 3,662,696 whereas reference may be had to U.S. Pat. No. 4,145,033 for a disclosure of a cowpers. In order to operate the furnace at maximum efficiency, a variable high pressure must be maintained therein and the temperature of the "hot blast" must be accurately controlled. The mixing station will typically proportion the relatively cool output of the booster with the hot air being furnished by the cowpers to thereby maintain the desired air temperature in the circular conduit and tuyere stocks.
In the interest of maintaining the reduction process within the furnace, it is common practice to add a fuel to the hot air blast. The fuel is typically injected into the tuyere stocks immediately upstream of the injection nozzles. In the prior art, because of the ease of handling and the abundance and comparatively modest cost thereof, it has been customary to add fuel in liquid form, particularly petroleum, to the hot air blast. A particular advantage incident to the use of a liquid fuel such as oil resides in the fact that its injection into the hot air blast does not have any significant disturbing effect on the temperature of the air.
In view of the increasing costs and uncertain future availability of suitable liquid fuels, the replacement of such liquid fuels by other combustible materials, and particularly materials such as coal which are in abundant supply, has become of the utmost importance. It is recognized that these solid fuels have the same ability as the liquid fuels for maintenance of the reduction process in a blast furnace. However, there are many serious obstacles to a change from the use of liquid fuels, such as oil, to solid fuels, such as pulverized coal. These obstacles include, but are not limited to, the difficulty in metering the quantity of fuel injected into the furnace and in preventing the fuel stream, and particularly the transport air for the pulverized solid fuel, from having a deleterious effect on the temperature of the hot air blast. A further complication resides in the necessity of raising the pressure of the fuel stream comprising the pulverized solid material to a level above that being maintained within the furnace whereby injection is possible.
A first of the above-discussed problems, particularly the manner of pressurizing the pulverized material containing fuel stream to a level above the pressure being maintained within the furnace, has been solved by the technique and apparatus disclosed in copending application Ser. No. 158,612, filed June 11, 1980 and assigned to the assignee of the present invention. Application Ser. No. 158,612 is incorporated herein by reference.
The process and apparatus of above-referenced application Ser. No. 158,612 does not solve the problem of the disturbance of the temperature of the hot air blast by the carrier or transport gas comprising the pulverized material containing fuel stream. This transport gas must, of course, be at a comparatively low temperature to insure that there will be no ignition of the pulverized combustible material within the delivery system. The latter problem, however, is addressed and solved by the process and apparatus disclosed in copending U.S. patent application Ser. No. 166,618 filed July 17, 1980 and also assigned to the assignee of the present invention. Application Ser. No. 166,618 is also incorporated herein by reference. In the process of said application Ser. No. 166,618, some of the "cold" air intended for the mixing station downstream of the cowper stoves is employed as the transport air for the pulverized solid fuel. Accordingly, the total quantity of "cold" air ultimately mixed with the hot blast in order to obtain a constant hot blast temperature does not vary. Rather, the mixing of the "cold" air with the hot air occurs at two stations; i.e., at the conventional mixing station and at the point of injection of the solid fuel into the blast pipes or tuyeres.
Continuing to refer to copending application Ser. No. 166,618, it was recognized that a loss in pressure would be encountered in the pneumatic feed line for the fuel stream. In the invention of application Ser. No. 166,618, it is proposed to employ a booster compressor, positioned upstream of the point of addition of the pulverized solid fuel, in order to compensate for this pressure loss and to thereby insure that the mixture of transport gas and pulverized fuel would be at a sufficiently high pressure to insure the injection thereof into the furnace against the counter pressure prevailing within the furnace. Since the pressure in a typical blast furnace is not constant, and in fact may vary over a range of 1.5 bar, there is obviously a problem presented in exercising proper control over the output pressure of the booster. A first possible solution to the problem discussed immediately above would be to cause the booster to constantly provide an output pressure commensurate with the extreme operating conditions and particularly with the maximum pressure possible within the furnace with the maximum fuel requirement. From an economic viewpoint, however, this is an unacceptable solution since constantly operating the booster at a level commensurate with the extreme operating conditions would result in a considerable waste of energy with the furnace operating at other than the said extreme conditions and this waste of energy would have a serious detrimental effect on the cost price per ton of the cast iron produced.
A second apparent solution would be to control the operation of the booster as a function of the pressure variations within the furnace; the furnace pressure being constantly monitored and thus known. It has, however, been discovered that the fuel stream pressure loss is a function of both the density of the fuel stream; i.e., the quantity of pulverized solid fuel per unit of transport gas; and the pressure prevailing inside the furnace. Further, the fuel stream pressure drop is a non-linear function of furnace pressure. Accordingly, the second possible solution is not viable since the exercise of control over the booster so as to insure the minimum fuel stream pressure required for injection of the fuel into the furnace, in the face of the non-linearity of the ratio between the fuel stream pressure loss and the pressure fluctuations inside the furnace, is at best an exceedingly complicated task.